1. Technical Field
The present invention relates to a motor including a sliding bearing formed by an outer surface of a rotatable shaft inserted through a sleeve and an inner surface of the sleeve.
2. Description of the Related Art
Many electronic devices include a fan motor or a motor for rotating a disk-shaped storage medium. Some of motors that can be used in the electronic devices include a sliding bearing. For example, the sliding bearing is formed by a sleeve and a shaft inserted into the sleeve. The sleeve supports the shaft in a rotatable manner around a rotation axis. A thrust plate provided at a bottom of the sleeve supports a lower end of the shaft in a slidable manner thereon. In another structure, the shaft is magnetically supported in an axial direction parallel to the rotation axis, that is, is supported in a non-contact manner.
In the above bearing structure, a rotor of the motor, including the shaft, can move in the axial direction with respect to the sleeve. This axial movement of the rotor makes rotation of the rotor unstable, causing generation of noises or sliding loss. In order to avoid this trouble, there are proposed various arrangements for restricting the axial movement of the rotor.
In one exemplary arrangement, a magnet is provided on a rear face of the thrust plate, i.e., a face of the thrust plate opposite to the face on which the shaft is slidable. The magnet magnetically attracts the shaft formed of magnetic material toward the thrust plate. (See Japanese Unexamined Patent Publication No. H09-317755, for example.)
In another exemplary arrangement that is similar to the above arrangement, a cup-shaped back yoke formed of magnetic material is provided in addition to the magnetically attracting magnet. (See Japanese Unexamined Patent Publication No. 2000-245101, for example.) This arrangement is advantageous in that a satisfactory magnitude of a magnetically attractive force can be obtained even by an inexpensive magnet and a leakage flux from the magnetically attracting magnet can be prevented.
In the latter arrangement, the back yoke is detachably fitted and fixed to a bearing holder so as to allow adjustment of the magnetically attractive force. Therefore, when an excessively large impact is applied downward from the shaft, the back yoke may be separated from the bearing holder. For this reason, applications of motors using the latter arrangement are limited to the use in environments in which no impact is applied to the motors from outside, or those motors or electronic devices employing those motors have to include an impact-resistant structure that can prevent application of impact to the back yoke, thus making motor structures complicated. Due to the above, the latter arrangement for magnetic attraction using the back yoke has not been widely used.